The present invention relates to a piezoelectric polymer composite material that can be used in an electroacoustic conversion film or the like for use in a speaker, a microphone, or the like.
Development is currently proceeding on flexible displays such as organic electroluminescent displays that utilize flexible substrates like plastic.
When this type of flexible display is used as part of a dual image display/audio playback device that displays images as well as plays audio, such as a television receiver, a speaker (an acoustic device) is required to generate the audio.
Conventional examples of speaker shapes typically include a so-called cone speaker in the form of a funnel and dome speakers in the form of a sphere. However, attempting to incorporate these speakers into the abovementioned type of flexible display generally impairs some of the inherent advantages of flexible displays, such as light weight and flexibility. Moreover, using external speakers makes the overall device more difficult to carry or move, makes it more difficult to set up the device on curved walls, and negatively affects the aesthetic appearance of the device.
To solve these problems, JP 2008-294493 A, for example, discloses using flexible sheet-shaped piezoelectric films to provide speakers that can be integrated into a flexible display without negatively affecting the light weight or flexibility of the display.
A piezoelectric film refers to a film created by poling a uniaxially stretched polyvinylidene diflouride (PVDF) film at high voltage. The resulting piezoelectric film exhibits the property of expanding or contracting in response to applied voltages.
Consider a portable flexible display that has a rectangular planar shape into which a speaker made from a piezoelectric film is integrated, that can be held in a gently bent state like print materials such as newspapers or magazines, and in which the screen display can be switched between a vertical and a horizontal orientation. In such a display, the image display surface is preferably able to bend not only in the vertical direction but also in horizontal direction.
However, the piezoelectric properties of a piezoelectric film made from uniaxially stretched PVDF exhibit in-plane anisotropy, and therefore, sound quality can differ significantly for a given curvature depending on the bending direction.
One example of a piezoelectric material that can be formed in a flexible sheet shape and in which the piezoelectric properties do not exhibit in-plane anisotropy, thereby solving the abovementioned problem, is a piezoelectric polymer composite material in which piezoelectric particles are dispersed in a polymer matrix.
KITAYAMA Toyoki, 1971 National Conference Proceedings of the Institute of Electronics, Information and Communication Engineers, 366 (1971), for example, discloses a piezoelectric polymer composite material produced by mixing powdered PZT ceramic (a piezoelectric material) with PVDF using a solvent casting process or a heated mixing process. The resulting piezoelectric polymer composite material retains both the flexibility of the PVDF and the high piezoelectric properties of the PZT ceramic.
In such a piezoelectric polymer composite material, the ratio of piezoelectric particles relative to the matrix is preferably increased in order to enhance the piezoelectric properties (that is, transmitting efficiency). However, piezoelectric polymer composite materials tend to become harder and more brittle as the amount of piezoelectric particles relative to the matrix increases.
SHIRAI Seiichi, NOMURA Hiroaki, OGA Juro, YAMADA Takeshi, OGUCHI Nobuki, Institute of Electronics, Information and Communication Engineers Technical Report, 24, 15 (1980) discloses adding a fluororubber to the PVDF in the piezoelectric polymer composite material disclosed in KITAYAMA Toyoki, 1971 National Conference Proceedings of the Institute of Electronics, Information and Communication Engineers, 366 (1971) in order to preserve the flexibility of the material and thereby solve the abovementioned problem.